1. Field of the Invention
This invention relates to an apparatus and method for automatically mounting chip-type electronic components (hereinafter referred to as "electronic component") on printed circuit boards.
2. Description of the Prior Art
An electronic component mounting apparatus typically includes a supply mechanism for supplying electronic components to a predetermined position; a table for supporting printed circuit boards thereon and adapted to be moved in X and Y directions by means of a drive mechanism; and a mounting head adapted to be moved between the predetermined position and a position above the table and moved vertically with respect to the predetermined position and the printed circuit boards on the table by a drive mechanism, connected to air-absorbing source and for holding via suction the electronic component, supplied by the supply mechanism, one by one at the predetermined position to mount the electronic components on the printed circuit boards supported by the table.
One of the considerations of a useful mounting apparatus is to achieve a precision placement of an electronic component at a predetermined position on a printed circuit board. Unfortunately, in the conventional mounting apparatus, there is a possibility that an electronic component will be placed on a printed circuit board in a wrong posture and/or in a state in which they are positionally deviated from a predetermined position on printed circuit board because the failure in suction of the electronic component in a correct position or posture may occur during the operation of sucking-up the electronic component by means of the mounting head and/or failure in the accurate positioning of the printed circuit board, which is supported on the table, may occur during the movement of the table. Therefore, it is necessary to precisely orient an electronic component before the electronic component is placed on a printed circuit board by means of the mounting head, and control the movement of the table in order to perform the precise positioning of the printed circuit board. For this purpose, some of the conventional mounting apparatuses further include a first camera for picking up a picture of an electronic component held by mounting head, a drive mechanism provided on the mounting head and including a pulse motor and the like and adapted for turning the mounting head, holding an electronic component via suction, in such a direction so as to orient the electronic component; a second camera for picking up a picture of a positioning mark or a fiducial mark, e.g., a through-hole or the like, the positioning mark being previously applied onto a printed circuit board on the X-Y table; and a picture processing unit previously storing therein data on pictures of an electronic component and a positioning mark of a printed circuit board when an electronic component is precisely positioned in a component mounting position of a printed circuit board, the picture processing unit receiving the pictures picked up by the first and second cameras to carry out comparison between the stored picture data and actual pictures obtained by the first and second cameras and send commands to the drive mechanism for turning the mounting head and the drive mechanism for movement of the X-Y table in dependence upon the results of the comparison. According to the commands the mounting head is turned in the .theta. direction by its drive mechanism in such a manner that an electronic component held by the mounting head is precisely oriented, and the movement of the X-Y table is controlled by the drive mechanism for the X-Y table in a such manner that a printed circuit board is accurately positioned. However, in the conventional mounting apparatus, the mounting head is provided with the drive mechanism for turning the head in the .theta. direction in addition to the drive mechanism for moving the mounting head held an electronic component, supplied to a predetermined position by means of the electronic component supply mechanism, to the position above the X-Y table, so that the mounting head is complex in construction and is rather heavy. Thus, a supporting mechanism for supporting the mounting head becomes large-sized.
Also, in some of the conventional mounting apparatuses, positioning of an electronic component held by the mounting head is performed by inputting into a computor data on coordinates of an electronic component mounting position on a printed circuit board to carry out numeric control. With this method, when a position of a printed circuit board on which a conducting pattern is actually formed is deviated from a position of a conducting preset according to a circuit diagram, an electronic component will be placed on the printed circuit board with electrodes of the electronic component in a position deviated from the conducting pattern on the printed circuit board. Also, when an electronic component treated by the mounting head has leads projecting therefrom and the leads have any defect, e.g., deformation or bending, the mounting head can not precisely locate the electronic component on a printed circuit board.
In the automatic mounting apparatus of this kind, a plurality of mounting heads are required in order to successively mount various kinds of electronic components on a printed circuit board. In order to meet such requirement, some devices of the forgoing type have a plurality of mounting heads adapted to be removably attached to a body for supporting the mounting heads. Tip ends of the mounting heads are different from one another in diameter depending upon dimensions of electronic components which are to be treated by the mounting apparatus. Each time the apparatus treats different electronic components, an operator installs mounting heads, having tip ends of diameters suitable for treating the electronic components, on the body for supporting the mounting heads. However, this requires substantial time, resulting in lower efficiency in mounting of the electronic components on printed circuit boards.
An example of an electronic component mounting system performed by an automatic mounting apparatus having a plurality of mounting heads is found, for example, in Japanese Patent Provisionary Publication No. 167802/1986. As shown in FIG. 1, this automatic mounting apparatus includes a rotating disk 1 adapted to intermittently rotate and a plurality of mounting heads 2 provided with respect to the rotating disk 1. The mounting heads can be arranged along a periphery of the rotating disk 1 at regular intervals and electronic components are adapted to be successively mounted on printed circuit boards by sequential movement of the mounting heads. More particularly, in the conventional mounting system, an electronic component supplied by an electronic component supply mechanism (not shown) is sucked up at a position G by means of one of the mounting heads 2, the electronic component held by the mounting head is then subjected to inspecting (inspecting a posture of the electronic component sucked up by the mounting head, and leads of the electronic component) through an image processing when the mounting head 2 holding the electronic component reaches a position H upon the intermittent rotation of the rotating disk 1, and the mounting head 2 is actuated to place the electronic component in a predetermined mounting position of a printed circuit board (not shown) when the mounting head reaches a position I upon further intermittent rotation of the rotating disk 1. However, in the conventional mounting system shown in FIG. 1, even though the mounting apparatus has a plurality of the mounting heads as described above, the movement of each of the mounting heads is limited to sequential movement, so that the conventional mounting system is undesirable from the point of view of improvement in the efficiency of the mounting operation.